I. Field of the Invention
The present invention relates to a particular structure for a particle detector crystal and a particle detector assembly incorporating a plurality of those particle detector crystals.
II. Background Information
Standard Anger scintillation camera assemblies are known which employ a lead collimator, a plurality of scintillation crystals, preferably comprised of NaI(Tl), and a plurality of photomultiplier tubes, with each crystal having a corresponding photomultiplier tube. Such cameras are employed to detect the emission of atomic particles, typically in the form of gamma ray photons, from a radioactive material which typically is injected in diagnostic amounts into a human body or the like. Each such particle emission is considered an event. By using a plurality of individual crystals, a percentage of such events may be detected by one crystal without necessarily affecting the capacity of the camera to detect a simultaneous event with another crystal.
At one point those skilled in the art believed that if a particle struck a particular section of a scintillation crystal the resulting light given off throughout that crystal would be proportional to the location of the incident particle. However, in practice light was found not to be proportionately distributed due to internal bounce. Accordingly, the prior art developed a particle detector crystal which employed a plurality of cuts which extended partially through the crystal to prevent internal bounce within the crystal, thereby effectively segregating the crystal into areas defined by those cuts. For example, U.S. Pat. No. 4,267,452 issued to Govaert discloses one such particle detector crystal employing cuts. The contents of Govaert are hereby expressly incorporated herein by reference.
When employing particle detector crystals having slots or cuts therein to reduce internal light bounce, photodetectors may be arranged in connection with such crystals so as to permit the section of the crystal to be identified within which a light producing event occurred. For example, in FIG. 1 there is illustrated a plurality of particle detector crystals 10a-d each of which comprises a crystal scintillation material which produces light when struck with a particle. Each crystal 10a-d is shaped with substantially parallel upper and lower surfaces 12a-d and 14a-d, respectively, which, as illustrated in FIG. 1, are each rectangularly shaped. An interconnecting sidewall is illustrated in FIG. 1 for each of crystals 10a-d in order to shape each crystal in the form of a solid rectangle.
In the absence of any slot within such crystals, a particle striking any portion of lower surfaces 12a-d would result in a light producing event in a corresponding position within crystal 10a-d, but the resulting light, due to internal bounce, would dissipate throughout the crystal and be released in approximately uniform intensity throughout upper surfaces 14a-d.
In order to reduce such internal light bounce so that resulting light from upper surfaces 14a-d is given off proportional to the location of an event within crystals 10a-d, the prior art, as evidenced by Govaert, employed at least one slot 16a-d in each crystal 10a-d extending from lower surfaces 12a-d into the interior of crystals 10a-d in a direction substantially parallel to an end wall of each of crystals 10a-d. As noted in Govaert, if crystals of approximately four centimeter height are employed, slots 16a-d are preferably on the order of three centimeters in height and approximately one to two millimeters in width. In addition, to further minimize distribution of light bounce within crystals 10a-d, each of slots 16a-d, may be filled with a light reflection material. Thus, slots 16a-d operate to effectively divide lower surfaces 12a-d into sections 18a-d and 20a-d, respectively.
Although not illustrated in FIG. 1, upper surfaces 14a-d may also be formed to incorporate exit windows of the type disclosed in Govaert which correspond one to each of sections 18a-d and 20a-d.
Crystals 10a-d of FIG. 1, accordingly, operate to provide light output from upper surfaces 14a-d which is distributed over those surfaces as a function of whether or not the particles causing the corresponding light producing event was incident in sections 18a-d or sections 20a-d of lower surfaces 12a-d. This discrimination in resultant light intensity from upper surfaces 14a-d, when detected, provides some degree of resolution for locating a light producing event within the volume defined by crystals 10a-d.
To detect light emitted from upper surfaces 14a-d, a plurality of photomultiplier tubes are employed in conventional particle detector assemblies. These photomultiplier tubes are typically circular in cross-sectional area and, when used in conjunction with particle detector crystals having slots formed therein to reduce internal light bounce, such photomultiplier tubes are typically staggered in location across two or more such detectors.
For example, as illustrated in FIG. 1 a photomultiplier tube 22a is located to extend over sections 18a of crystal 10a, 20b of crystal 10b, 20d of crystal 10d, and 18c of crystal 10c. This arrangement is further illustrated in FIG. 2 which shows an bottom view of the portion of a particle detector assembly employing particle detector crystals and photomultiplier tubes as illustrated in FIG. 1. Thus, as is illustrated in FIGS. 1 and 2, the center of a photomultiplier tube 22a is located at the common corner of sections 18a, 20b, 20d and 18c of crystals 10a, 10b, 10d and 10c, respectively. Adjacent photomultipler tubes 22b-g are similarly arranged with regard to corresponding particle detector crystals. Thus, photo multiplier tubes 22a-g are arranged in a shifted square packing array as illustrated in FIG. 2.
In operation, an event occuring, for example, in crystal 10a as a result of a particle entering section 18a of lower surface 12a would result in the production of light that would be confined to that section of crystal 10a adjacent to lower section 18a by operation of slot 16a. Thus, light emitted through upper section 14a of crystal 10a as a result of that event would have greater intensity in that section of upper surface 14a corresponding to section 18a than that section of upper surface 14a corresponding to section 20a. Accordingly, photomultiplier tube 22a would have a greater light output as a result of that event than photomultiplier tube 22e which has a quadrant aligned over section 20a of crystal 10a as shown in FIG. 2.
Through detection of at least some light in photomultiplier tube 22e and a greater amount of light in photomultiplier tube 22a, a determination may be made that the light producing event occurred in that section of detector 10a corresponding to section 18a, since section 18a of crystal 10a is the only common section for photomultiplier tubes 22a and 22e. If, for example, the light intensity were reversed, that is to say greater light were received in photomultiplier tube 22e than in photomultiplier tube 22a, this relationship of a light received would result in a determination of light producing event occurring in section 20a of crystal 10a.
In a similar manner, the location of light producing events may be determined throughout an array comprising crystals 10a-d and the like and corresponding photomultiplier tubes 22a-g and the like.
Although such conventional particle detector crystals and corresponding particle detector assemblies provide an improvement over particle detector crystals which do not employ internal light bounce impeding slots, such conventional detectors and corresponding assemblies, nevertheless, have the disadvantage of having a large dead volume between photomultiplier tubes in which light emitted from corresponding sections of upper surfaces 14a-d is not detected. For example, as illustrated in FIG. 2, dead areas 24 exist between photomultiplier tubes 22a-g. These dead areas 24 occur in a square packing or shifted square packing arrangement of photomultiplier tubes 22a-g.
It is, accordingly, an object of the present invention to improve the efficiency of known prior art particle detector crystals and associated particle detector assemblies employing internal light bounce reducing slots.
Another object of the present invention is to improve spatial accuracy of known prior art detector crystals and associated particle detector assemblies.
Additional objects and advantages will be set forth in the description which follows and, in part, will be obvious from the description or may be learned by practice of the invention.